The invention relates to a method for the treatment of metallic materials, particularly for the consolidation of the structure or texture of metallic materials as well as metallic blanks.
Conventional treatment or respectively, reformation techniques for metallic materials provide for consolidation results, which generally are not totally satisfactory. Special metallic materials for example of the group of titanium aluminides or magnesium materials have, after the conventional treatment or transformation techniques such as forging or extrusion pressing, substantial chemical and structural inhomogeneities in their texture, which cannot be tolerated for certain technical applications. With the known treatment or, respectively, transformation techniques only relatively low transformation degrees can be achieved. This is not acceptable if the metallic materials are to be used for example in thermically and mechanically highly stressed areas for example for turbine blades of jet engines for airplanes or connecting rods for automobile engines.
Metallic materials such as inter-metallic titanium aluminides are very brittle and therefore hard to transform. In the past, such metallic materials were manufactured exclusively by melt metallurgical processes, mainly by vacuum arc melting, plasma melting and induction melting. Although the molten material is usually melted two or three times, the cast bodies still have substantial quality deficiencies, mainly coarse grain textures with certain preferred orientations of the crystals, large local variations in the composition and the occurrence of pores. Such deficiencies occur not only with the primary casting for example of titanium aluminides but also with many other metallic materials so that they are not suitable—as already mentioned—for the direct manufacture of components from the castings. The material, which is present as primary casting, must therefore be consolidated structurally and chemically. To this end, high temperature transformation by forging or extrusion pressing is generally used mainly for obtaining a fine-grain texture and a homogenization, that is, a reduction of the local variations of the material composition for example in metallic alloys.
In the past, the texture of the castings was consolidated by re-crystallization procedures and phase conversions, which were initiated by an input of mechanical energy into the material during the high-temperature transformation. The fineness and homogeneity of the material texture after transformation depends on the transformation temperature and the transformation velocity and to a large degree also on the transformation degree that is on the extent of the plastic deformation achieved during the transformation of the material. This transformation degree is limited with conventional one-step forging by compression generally to 90 to 95%. With such transformation degrees, high secondary tensions occur at the periphery of the forged body which often result in the formation of cracks. This is particularly critical with brittle materials such as titanium aluminides. These materials are therefore generally transformed to a much lesser degree. Higher degrees of transformation require multi-step forging which is expensive and time-consuming and is not usable for all desired component shapes.
It is also disadvantageous that no suitable die materials are available for forging at temperatures above 1000° C. The dies of molybdenum alloys used so far at temperatures up to 1000° C. can only be operated under a protective gas cover, which makes the forging a difficult task.
With the extrusion pressing, which has also been used for the transformation, substantially higher degrees of transformation can be achieved than with forging. With a superimposed hydrostatic compression also brittle materials can be transformed relatively well. In practical applications however, the transformation degree achieved by extrusion pressing is generally limited by the geometry of the desired body to a reduction in cross-section by about 10:1. It is also a disadvantage that substantially higher temperatures are required for the extrusion pressing than for the forging. Materials like titanium aluminides, which are subject to oxidation and corrosion must therefore be encapsulated for the extrusion pressing, which is complicated and expensive.
It is the object of the present invention to provide a method for the treatment of metallic materials which provides for a much improved consolidation of the texture and which is also applicable for very brittle materials, which has, so far, been difficult to transform such as inter-metallic alloys.